Welcome to the forum, you've come to the right place, a lot of knowledgeable people on here who are happy to share information and help. They've helped me a lot.
1. As F5 suggests, could be an air leak, sucking in air will really mess up the running. Places to check are reed block housing, main bearing seals especially the left side behind the flywheel as this runs dry on the outside, also cylinder base gasket, remember you said it's just had a new top end fitted.
2. Fuel starvation? Is the carb getting plenty of fuel? There's a gauze filter on the petrol tap inside the tank, remove/inspect/clean. Also check the fuel tank breather in the filler cap is working and isn't blocked, if no air's getting into the tank, petrol won't flow out easily.
3. Is the tail pipe clear? I've seen tail pipes blocked with wadding that's come loose. Bikes start but won't rev.
4. Check operation of float valve, is it flowing fuel freely? You don't mention fuel overflowing from the carb, so I assume the float valve is closing/sealing properly. Check float height before adjusting any jetting.
5. You didn't mention it but I assume you have a freshly cleaned air filter.
6. Choke not closing properly? Is the choke cable operated or just a pull knob on the carb? Cable operated chokes can stick open a bit, check. Also remove the choke plunger and inspect the rubber seal on the end of it.
I haven't listed these in any particular order, just as they came into my head, best to check the easiest/simple things first.
Once you fix or improve things, be careful running the bike on a 160 main jet, could be pretty lean.
I don't think this is related to your problem... but premix oil at 100:1 Are you sure that's correct? I think most people mix 100% synthetic oil at 50:1.
Ahhhh, I've just looked up Amsoil Saber, it's designed for garden power tools such as chainsaws, that's what I read. I think you should be using Amsoil Interceptor or Amsoil Dominator at 50:1 or another brand of 100% synthetic oil designed for competition 2 stroke engines.
Good luck, hope you get it sorted. Please let us know how you get on.